ath9k: replace snprintf() with scnprintf()

Whenever the return value of snprintf() is used to calculate
remaining buffer-space, we wanted to use sncprintf() instead.

Indentation is adapted where possible. Some lines exceed the
line width limit, either they did it already before, or
since they can not be broken reasonably well.

Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>